1. Field of the Invention
The present invention relates to an electronic keyboard musical instrument which is placed on a supporting surface when it is used.
2. Description of the Related Art
Japanese Patent Application KOKAI Publication No. 2000-81885 discloses an electronic keyboard musical instrument which is placed on a supporting surface such as a floor of a building when it is use. The electronic keyboard musical instrument comprises an instrument main unit 11 which includes a keyboard having a plurality of keys and which generates electrical signals in response to depressions of the keys of the keyboard; speakers 21 which produce sounds corresponding to the electrical signals from the instrument main unit; and an instrument main unit supporting member which is placed on the supporting surface and which supports the instrument main unit 11 at a position apart upward from the supporting surface. The instrument main unit supporting member includes: a main unit housing 10 which has an outer surface facing an external space and an internal space surrounded by the outer surface; and a pair of side plates 30 which is placed on the supporting surface and which supports the main unit housing 10 at a position apart upward from the supporting surface. And, the instrument main unit 11 is housed in the internal space of the main unit housing 10.
A lower surface region of the outer surface of the main unit housing 10, facing the supporting surface, is provided by a shelf plate 12 of the main unit housing 10. The shelf plate 12 is provided with openings 12a in a rearward of the instrument main unit 11, and the openings 12a make the internal space of the main unit housing 10 being communicate with the external space. The speakers 21 are located below the openings 12a and housed in a speaker box 20 supported by the side plates 30. In the speaker box 20, a front side of each speaker 21 faces obliquely downward and forward, and a rear side of each speaker 21 faces each opening 12a. 
In such the electronic keyboard musical instrument disclosed in the Japanese Patent Application KOKAI Publication No. 2000-81885, sounds which are produced by the speakers 21 when a player facing the keyboard presses the keys of the keyboard are reflected by the supporting surface facing the front sides of the speakers 21 so that the player hears the reflected sounds from the feet of the player, and at the same time, the player hears the sounds from the internal space of the main unit housing 10 through the openings 12a of the outer surface of the main unit housing 10 facing the rear sides of the speakers 21.
The front side of each speaker 21 transmits the sound produced by each speaker 21 most effectively, but the sound from the front side of each speaker 21 is always reflected by the supporting surface so that a quality of the sound is degraded. Since a means for driving a diaphragm of the speaker 21 is provided on the rear side of the speaker 21, a quality of the sound from the rear side of the speaker 21 is degraded in comparison with the quality of the sound from the front side of the speaker 21. Further, in the electronic keyboard musical instrument of the Japanese Patent Application KOKAI Publication No. 2000-81885, the rear side of each speaker 21 is located apart downward from each opening 12a in the outer surface of the main unit housing 10. Therefore, the quality of the sound transmitted from the rear side of each speaker to the internal space of the main unit housing 10 through each opening 12a is further degraded in comparison with the quality of the sound from the front side of each speaker 21.
The speaker box 20 which is supported by the side plates 30 below the main unit housing 10 and which houses the speakers 21 complicates the structure of the instrument main unit supporting member, increases the weight of the instrument main unit supporting member, and increases the manufacturing cost of the instrument main unit supporting member.
Japanese Patent Application KOKAI Publication No. 8-54880 also discloses an electronic keyboard musical instrument which is placed on a supporting surface when it is used. The electronic keyboard musical instrument comprises an instrument main unit which includes a keyboard 12 having a plurality of keys and which generates electrical signals in response to depressions of the keys of the keyboard; three speakers 14A, 14B and 15 which produce sounds corresponding to the electrical signals from the instrument main unit; and an instrument main unit supporting member which includes upper and lower parts 10 and 11, which are placed on a supporting surface 20 such as an upper surface of a table and which support the instrument main unit at a position apart upward from the supporting surface 20.
The upper part of the instrument main unit supporting member comprises a main unit housing 10 which includes an outer surface facing an external space, and an internal space which is surrounded by the outer surface and which houses the instrument main unit. The lower part 11 of the instrument main unit supporting member comprises a plurality of side plates 17 which is placed on the supporting surface 20 and which supports the main unit housing 10 at a position apart upward from the supporting surface 20.
Three speakers 14A, 14B and 15B are housed in the internal space of the main unit housing 10. In the internal space, two speakers 14A and 14B are fixed to an inner surface of the internal space with their front sides facing two openings formed in an upper surface region of the outer surface of the main unit housing 10, and the keyboard 12 is exposed in the upper surface region. A lower surface region of the outer surface of the main unit housing 10, facing the supporting surface 20, is provided by a bottom plate 16 of the main unit housing 10. Further in the internal space, one speaker 15 is fixed to the inner surface of the internal space with its front side facing one opening formed in the bottom plate 16.
A manufacturing cost of the electronic keyboard musical instrument of Japanese Patent Application KOKAI Publication No. 8-54880 is increased by using the speakers which transmit sounds only in directions in which the front sides of the speakers face from the outer surface of the main unit housing 10 of the instrument main unit supporting member, like the speakers 14A and 14B whose front sides face upward and the speaker 15 whose front side faces downward.
Further, the main unit housing 10 having the internal space in which the speakers 14A, 14B and 15 and the instrument main unit including the keyboard 12 are housed has large outer dimensions, and a weight of the main unit housing 10 is heavy.
Japanese Utility Model Application KOKAI Publication No. 3-130697 discloses an instrument main unit supporting member for an electronic keyboard musical instrument, which is placed on a supporting surface like a floor of a building when the electronic keyboard musical instrument is used. The instrument main unit supporting member comprises a shelf plate 1 at a position apart upward from a supporting surface, and an instrument main unit which includes a keyboard having a plurality of keys and which generates electrical signals in response to depressions of the keys of the keyboard is placed on the shelf plate 1. The instrument main unit supporting member supports a speaker box 2 adjacent to a lower surface region of the shelf plate 1, the lower surface region facing the supporting surface. The speaker box 2 houses two speakers 6 in its internal space with their front sides facing the lower surface region, and two openings 8 are formed in the shelf plate 1 to correspond to the front sides of the speakers 6. Ducts 7 for bass sounds are opened in a front surface region of the outer surface of the speaker box 2, the front surface region facing a player of the electronic keyboard musical instrument.
The speaker box 2 housing two speakers 6 and 6 in its internal space complicates a structure of the instrument main unit supporting member and increases outer dimensions of the instrument main unit supporting member.